The purpose of this study is to determine whether some or all of the metabolic alterations in red cell glycolytic intermediates and enzymes that are characteristic of erythrocytes from term and premature infants are intrinsic to the fetal erythrocyte, and follow a normal developmental pattern, or are influenced by plasma factors. The glycolytic enzymes and intermediates in healthy premature and full-term newborns will be followed during the first year of life to establish when these values approach those observed in adults and to detect differences, if any, between the two groups. These alterations will then be compared with possible modifying factors such as the percent fetal hemoglobin red cell age, aging characteristics of red cell enzymes, intracellular pH and plasma phosphate concentration. Glycolytic enzymes and intermediates, intracellular pH and plasma phosphate concentration will also be followed sequentially in the developing fetal lamb in an attempt to establish an animal model.